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Origin and Structures of Solar Eruptions I: Magnetic Flux Rope (Invited Review)  

Xin Cheng   Submitted: 2017-05-23 19:06

Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1-3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.

Authors: X. Cheng, Y. Guo, M. D. Ding
Projects: None

Publication Status: 46 pages, 9 figures, Accepted by SCIENCE CHINA Earth Sciences
Last Modified: 2017-05-24 14:22
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On the Characteristics of Footpoints of Solar Magnetic Flux Ropes during the Eruption  

Xin Cheng   Submitted: 2016-05-15 23:08

We investigate the footpoints of four erupted magnetic flux ropes (MFRs) that appear as sigmoidal hot channels prior to the eruptions in the Atmospheric Imaging Assembly high temperaure passbands. The simultaneous Helioseismic and Magnetic Imager observations disclose that one footpoint of the MFRs originates in the penumbra or penumbra edge with a stronger magnetic field, while the other in the moss region with a weaker magnetic field. The significant deviation of the axis of the MFRs from the main polarity inversion lines and associated filaments suggests that the MFRs have ascended to a high altitude, thus being distinguishable from the source sigmoidal ARs. The more interesting thing is that, with the eruption of the MFRs, the average inclination angle and direct current at the footpoints with stronger magnetic field tend to decrease, which is suggestive of a straightening and untwisting of the magnetic field in the MFR legs. Moreover, the associated flare ribbons also display an interesting evolution. They initially appear as sporadical brightenings at the two footpoints of and in the regions below the MFRs and then quickly extend to two slender sheared J-shaped ribbons with the two hooks corresponding to the two ends of the MFRs. Finally, the straight parts of the two ribbons separate from each other, evolving into two widened parallel ones. These features mostly conforms to and supports the recently proposed three-dimensional standard CME/flare model, i.e., the twisted MFR eruption stretches and leads to the reconnection of the overlying field that transits from a strong to weak shear with the increasing height.

Authors: X. Cheng and M. D. Ding
Projects: SDO-HMI

Publication Status: Accepted for publication in ApJ Supplement Series
Last Modified: 2016-05-16 09:09
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Spectroscopic Diagnostics of Solar Magnetic Flux Ropes Using Iron Forbidden Line  

Xin Cheng   Submitted: 2016-05-03 18:13

In this Letter, we present Interface Region Imaging Spectrograph Fe XXI 1354.08 Å forbidden line emission of two magnetic flux ropes (MFRs) that caused two fast coronal mass ejections with velocities of ≥1000 km s-1 and strong flares (X1.6 and M6.5) on 2014 September 10 and 2015 June 22, respectively. The EUV images at the 131 Å and 94 Å passbands provided by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory reveal that both MFRs initially appear as suspended hot channel-like structures. Interestingly, part of the MFRs is also visible in the Fe XXI 1354.08 forbidden line, even prior to the eruption, e.g., for the SOL2014-09-10 event. However, the line emission is very weak and that only appears at a few locations but not the whole structure of the MFRs. This implies that the MFRs could be comprised of different threads with different temperatures and densities, based on the fact that the formation of the Fe XXI forbidden line requires a critical temperature (∼11.5 MK) and density. Moreover, the line shows a non-thermal broadening and a blueshift in the early phase. It suggests that magnetic reconnection at that time has initiated; it not only heats the MFR and, at the same time, produces a non-thermal broadening of the Fe XXI line but also produces the poloidal flux, leading to the ascending of the MFRs.

Authors: X. Cheng and M. D. Ding
Projects: None

Publication Status: 6 pages, 4 figures, accepted for publication in ApJ Letters
Last Modified: 2016-05-04 09:22
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A Two-ribbon White-light Flare Associated with a Failed Solar Eruption Observed by ONSET, SDO, and IRIS  

Xin Cheng   Submitted: 2015-07-08 23:25

Two-ribbon brightenings are one of the most remarkable characteristics of an eruptive solar flare and are often used for predicting the occurrence of coronal mass ejections (CMEs). Nevertheless, it was called in question recently whether all two-ribbon flares are eruptive. In this paper, we investigate a two ribbon-like white-light (WL) flare that is associated with a failed magnetic flux rope (MFR) eruption on 2015 January 13, which has no accompanying CME in the WL coronagraph. Observations by Optical and Near-infrared Solar Eruption Tracer and Solar Dynamics Observatory reveal that, with the increase of the flare emission and the acceleration of the unsuccessfully erupting MFR, two isolated kernels appear at the WL 3600 Å passband and quickly develop into two elongated ribbon-like structures. The evolution of the WL continuum enhancement is completely coincident in time with the variation of Fermi hard X-ray 26-50 keV flux. Increase of continuum emission is also clearly visible at the whole FUV and NUV passbands observed by Interface Region Imaging Spectrograph. Moreover, in one WL kernel, the Si IV, C II, and Mg II h/k lines display significant enhancement and non-thermal broadening. However, their Doppler velocity pattern is location-dependent. At the strongly bright pixels, these lines exhibit a blueshift; while at moderately bright ones, the lines are generally redshifted. These results show that the failed MFR eruption is also able to produce a two-ribbon flare and high-energy electrons that heat the lower atmosphere, causing the enhancement of the WL and FUV/NUV continuum emissions and chromospheric evaporation.

Authors: X. Cheng, Q. Hao, M. D. Ding, K. Liu, P. F. Chen, C. Fang, Y. D. Liu
Projects: None

Publication Status: accepted for publication in ApJ
Last Modified: 2015-07-10 20:37
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Extreme Ultraviolet Imaging of Three-dimensional Magnetic Reconnection in a Solar Eruption  

Xin Cheng   Submitted: 2015-06-26 22:18

Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively. It is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is studied in three dimensions (3D) using the combined perspectives of two spacecraft. The sequence of extreme ultraviolet (EUV) images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other, forming a separator or quasi-separator and then reconnecting. The plasma near the reconnection site is subsequently heated from ~1 to ≥5 MK. Shortly afterwards, warm flare loops (~3 MK) appear underneath the hot plasma. Other observational signatures of reconnection, including plasma inflows and downflows, are unambiguously revealed and quantitatively measured. These observations provide direct evidence of magnetic reconnection in a 3D configuration and reveal its origin.

Authors: J. Q. Sun, X. Cheng, M. D. Ding, Y. Guo, E. R. Priest, C. E. Parnell, S. J. Edwards, J. Zhang, P. F. Chen, C. Fang
Projects: None

Publication Status: Nature Communications 6, Article number: 7598 doi:10.1038/ncomms8598
Last Modified: 2015-06-29 11:55
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Predicting the Arrival Time of Coronal Mass Ejections with the Graduated Cylindrical Shell and Drag Force Model  

Xin Cheng   Submitted: 2015-05-05 19:29

Accurately predicting the arrival of coronal mass ejections (CMEs) at the Earth based on remote images is of critical significance in the study of space weather. In this paper, we make a statistical study of 21 Earth directed CMEs, exploring in particular the relationship between CME initial speeds and transit times. The initial speed of a CME is obtained by fitting the CME with the Graduated Cylindrical Shell model and is thus free of projection effects. We then use the drag force model to fit results of the transit time versus the initial speed. By adopting different drag regimes, i.e., the viscous, aerodynamics, and hybrid regimes, we get similar results, with the least mean estimation error of the hybrid model of 12.9 hours. CMEs with a propagation angle (the angle between the propagation direction and the Sun-Earth line) larger than its half angular width arrive at the Earth with an angular deviation caused by factors other than the radial solar wind drag. The drag force model cannot be well applied to such events. If we exclude these events in the sample, the prediction accuracy can be improved, i.e., the estimation error reduces to 6.8 hours. This work suggests that it is viable to predict the arrival time of CMEs at the Earth based on the initial parameters with a fairly good accuracy. Thus, it provides a method of space weather forecast of 1-5 days following the occurrence of CMEs.

Authors: Tong Shi, Yikang Wang, Linfeng Wan, Xin Cheng, Mingde Ding, Jie Zhang
Projects: None

Publication Status: 9 pages, 7 figures, 3 tables, ApJ in press
Last Modified: 2015-05-06 10:09
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A Type II Radio Burst without a Coronal Mass Ejection  

Xin Cheng   Submitted: 2015-03-03 18:57

Type II radio bursts are thought to be a signature of coronal shocks. In this paper, we analyze a short-lived type II burst that started at 07:40 UT on 2011 February 28. By carefully checking white-light images, we find that the type II radio burst is not accompanied by a coronal mass ejection, only with a C2.4 class flare and narrow jet. However, in the extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we find a wave-like structure that propagated at a speed of ~600 km s-1 during the burst. The relationship between the type II radio burst and the wave-like structure is in particular explored. For this purpose, we first derive the density distribution under the wave by the differential emission measure (DEM) method, which is used to restrict the empirical density model. We then use the restricted density model to invert the speed of the shock that produces the observed frequency drift rate in the dynamic spectrum. The inverted shock speed is similar to the speed of the wave-like structure. This implies that the wave-like structure is most likely a coronal shock that produces the type II radio burst. We also examine the evolution of the magnetic field in the flare-associated active region and find continuous flux emergence and cancellation taking place near the flare site. Based on these facts, we propose a new mechanism for the formation of the type II radio burst, i.e., the expansion of the strongly-inclined magnetic loops after reconnected with nearby emerging flux acts as a piston to generate the shock wave.

Authors: Su, W., Cheng, X., Ding, M. D., Chen, P. F., and Sun, J. Q.
Projects: SDO-AIA

Publication Status: 18 pages, 10 figures, accepted for publication in ApJ
Last Modified: 2015-03-04 11:12
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Imaging and Spectroscopic Diagnostics on the Formation of Two Magnetic Flux Ropes Revealed by SDO/AIA and IRIS  

Xin Cheng   Submitted: 2015-03-02 05:13

Helical magnetic flux rope (MFR) is a fundamental structure of corona mass ejections (CMEs) and has been discovered recently to exist as a sigmoidal channel structure prior to its eruption in the extreme ultraviolet (EUV) high temperature passbands of the Atmospheric Imaging Assembly (AIA). However, when and where the MFR is built up are still elusive. In this paper, we investigate two MFRs (MFR1 and MFR2) in detail, whose eruptions produced two energetic solar flares and CMEs on 2014 April 18 and 2014 September 10, respectively. The AIA EUV images reveal that for a long time prior to their eruption, both MFR1 and MFR2 are under formation, which is probably through magnetic reconnection between two groups of sheared arcades driven by the shearing and converging flows in the photosphere near the polarity inversion line. At the footpoints of the MFR1, the Interface Region Imaging Spectrograph Si IV, C II, and Mg II lines exhibit weak to moderate redshifts and a non-thermal broadening in the pre-flare phase. However, a relatively large blueshift and an extremely strong non-thermal broadening are found at the formation site of the MFR2. These spectral features consolidate the proposition that the reconnection plays an important role in the formation of MFRs. For the MFR1, the reconnection outflow may propagate along its legs, penetrating into the transition region and the chromosphere at the footpoints. For the MFR2, the reconnection probably takes place in the lower atmosphere and results in the strong blueshift and non-thermal broadening for the Mg II, C II, and Si IV lines.

Authors: X. Cheng, M. D. Ding, C. Fang
Projects: IRIS

Publication Status: accepted for publication in ApJ
Last Modified: 2015-03-02 09:52
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On the Relationship Between a Hot-channel-like Solar Magnetic Flux Rope and its embedded Prominence  

Xin Cheng   Submitted: 2014-06-17 20:09

Magnetic flux rope (MFR) is a coherent and helical magnetic field structure that is recently found probably to appear as an elongated hot-channel prior to a solar eruption. In this paper, we investigate the relationship between the hot-channel and associated prominence through analyzing a limb event on 2011 September 12. In the early rise phase, the hot-channel was cospatial with the prominence initially. It then quickly expanded, resulting in a separation of the top of the hot-channel from that of the prominence. Meanwhile, both of them experienced an instantaneous morphology transformation from a Λ shape to a reversed-Y shape and the top of these two structures showed an exponential increase in height. These features are a good indication for the occurrence of the kink instability. Moreover, the onset of the kink instability is found to coincide in time with the impulsive enhancement of the flare emission underneath the hot-channel, suggesting that the ideal kink instability likely also plays an important role in triggering the fast flare reconnection besides initiating the impulsive acceleration of the hot-channel and distorting its morphology. We conclude that the hot-channel is most likely the MFR system and the prominence only corresponds to the cool materials that are collected in the bottom of the helical field lines of the MFR against the gravity.

Authors: X. Cheng, M. D. Ding, J. Zhang, A. K. Srivastava, Y. Guo, P. F. Chen, J. Q. Sun
Projects: SDO-AIA

Publication Status: accepted for publication in ApJ Letters
Last Modified: 2014-06-18 13:26
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Formation of a Double-decker Magnetic Flux Rope in the Sigmoidal Solar Active Region 11520  

Xin Cheng   Submitted: 2014-05-20 18:31

In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s-1. The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About two hours before the eruption, indications for a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region.

Authors: X. Cheng, M. D. Ding, J. Zhang, X. D. Sun, Y. Guo, Y. M. Wang, B. Kliem, Y. Y. Deng
Projects: None

Publication Status: accepted for publication in ApJ. 12 pages, 9 figures, and 1 table
Last Modified: 2014-05-21 13:36
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Differential Emission Measure Analysis of A Limb Solar Flare on 2012 July 19  

Xin Cheng   Submitted: 2014-03-26 02:45

We perform Differential Emission Measure (DEM) analysis of an M7.7 flare that occurred on 2012 July 19 and was well observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamic Observatory. Using the observational data with unprecedented high temporal and spatial resolution from six AIA coronal passbands, we calculate the DEM of the flare and derive the time series of maps of DEM-weighted temperature and emission measure (EM). It is found that, during the flare, the highest EM region is located in the flare loop top with a value varying between 8.4 1028 cm^-5 and 2.5 1030 cm^-5. The temperature there rises from 8 MK at about 04:40 UT (the initial rise phase) to a maximum value of 13 MK at about 05:20 UT (the hard X-ray peak). Moreover, we find a hot region that is above the flare loop top with a temperature even up to 16 MK. We also analyze the DEM properties of the reconnection site. The temperature and density there are not as high as that in the loop top and the flux rope, indicating that the main heating may not take place inside the reconnection site. In the end, we examine the dynamic behavior of the flare loops. Along the flare loop, both the temperature and the EM are the highest in the loop top and gradually decrease towards the footpoints. In the northern footpoint, an upward force appears with a biggest value in the impulsive phase, which we conjecture originates from chromospheric evaporation.

Authors: J. Q. Sun, X. Cheng, M. D. Ding
Projects: SDO-AIA

Publication Status: 13 pages, 10 figures. To appear in ApJ
Last Modified: 2014-03-27 00:15
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Tracking the Evolution of A Coherent Magnetic Flux Rope Continuously from the Inner to the Outer Corona  

Xin Cheng   Submitted: 2013-10-24 16:13

The magnetic flux rope (MFR) is believed to be the underlying magnetic structure of coronal mass ejections (CMEs). However, it remains unclear how an MFR evolves into and forms the multi-component structure of a CME. In this paper, we perform a comprehensive study of an extreme-ultraviolet (EUV) MFR eruption on 2013 May 22 by tracking its morphological evolution, studying its kinematics, and quantifying its thermal property. As EUV brightenings begin, the MFR starts to rise slowly and shows helical threads winding around an axis. Meanwhile, cool filamentary materials descend spirally down to the chromosphere. These features provide direct observational evidence of intrinsically helical structure of the MFR. Through detailed kinematical analysis, we find that the MFR evolution experiences two distinct phases: a slow rise phase and an impulsive acceleration phase. We attribute the first phase to the magnetic reconnection within the quasi-separatrix-layers surrounding the MFR, and the much more energetic second phase to the fast magnetic reconnection underneath the MFR. We suggest that the transition between these two phases be caused by the torus instability. Moreover, we identify that the MFR evolves smoothly into the outer corona and appears as a coherent structure within the white light CME volume. The MFR in the outer corona was enveloped by bright fronts that originated from plasma pile-up in front of the expanding MFR. The fronts are also associated with the preceding sheath region followed the outmost MFR-driven shock.

Authors: X. Cheng, M. D. Ding, Y. Guo, J. Zhang, A. Vourlidas, Y. D. Liu, O. Olmedo, J. Q. Sun, and C. Li
Projects: SDO-AIA

Publication Status: accepted by ApJ; any comments are welcome!
Last Modified: 2013-10-28 23:03
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Investigating Two Successive Flux Rope Eruptions In A Solar Active Region  

Xin Cheng   Submitted: 2013-04-28 05:46

We investigate two successive flux rope (FR1 and FR2) eruptions resulting in two coronal mass ejections (CMEs) on 2012 January 23. Both FRs appeared as an EUV channel structure in the images of high temperature passbands of the Atmospheric Imaging Assembly prior to the CME eruption. Through fitting their height evolution with a function consisting of linear and exponential components, we determine the onset time of the FR impulsive acceleration with high temporal accuracy for the first time. Using this onset time, we divide the evolution of the FRs in the low corona into two phases: a slow rise phase and an impulsive acceleration phase. In the slow rise phase of the FR1, the appearance of sporadic EUV and UV brightening and the strong shearing along the polarity inverse line indicates that the quasi-separatrix-layer reconnection likely initiates the slow rise. On the other hand for the FR2, we mainly contribute its slow rise to the FR1 eruption, which partially opened the overlying field and thus decreased the magnetic restriction. At the onset of the impulsive acceleration phase, the FR1 (FR2) reaches the critical height of 84.4pm11.2 Mm (86.2pm13.0 Mm) where the decline of the overlying field with height is fast enough to trigger the torus instability. After a very short interval (sim2 minutes), the flare emission began to enhance. These results reveal the compound activity involving multiple magnetic FRs and further suggest that the ideal torus instability probably plays the essential role of initiating the impulsive acceleration of CMEs.

Authors: X. Cheng, J. Zhang, M. D. Ding, O. Olmedo, X. D. Sun, Y. Guo & Y. Liu
Projects: SDO-AIA

Publication Status: 9 pages, 5 figures, Accepted for publication in ApJ Letters
Last Modified: 2013-04-30 12:03
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The Driver of Coronal Mass Ejections in the Low Corona: A Flux Rope  

Xin Cheng   Submitted: 2012-11-28 18:26

Recent Solar Dynamic Observatory observations reveal that coronal mass ejections (CMEs) consist of a multi-temperature structure: a hot flux rope and a cool leading front (LF). The flux rope first appears as a twisted hot channel in the Atmospheric Imaging Assembly 94 Å and 131 Å passbands. The twisted hot channel initially lies along the polarity inversion line and then rises and develops into the semi-circular flux rope-like structure during the impulsive acceleration phase of CMEs. In the meantime, the rising hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME LF. In this paper, we study in detail two well-observed CMEs occurred on 2011 March 7 and 2011 March 8, respectively. Each of them is associated with an M-class flare. Through a kinematic analysis we find that: (1) the hot channel rises earlier than the first appearance of the CME LF and the onset of the associated flare; (2) the speed of the hot channel is always faster than that of the LF, at least in the field of view of AIA. Thus, the hot channel acts as a continuous driver of the CME formation and eruption in the early acceleration phase. Subsequently, the two CMEs in white-light images can be well reproduced by the graduated cylindrical shell flux rope model. These results suggest that the pre-existing flux rope plays a key role in CME initiation and formation.

Authors: X. Cheng, J. Zhang, D. M. Ding, Y. Liu, W. Poomvises
Projects: SDO-AIA

Publication Status: 18 pages, 12 figures, 1 table, accepted for publication in ApJ
Last Modified: 2012-12-01 23:52
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Differential Emission Measure Analysis of Multiple Structural Components of Coronal Mass Ejections in the Inner Corona  

Xin Cheng   Submitted: 2012-10-26 22:52

In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis is based on the six-passband EUV observations of solar corona from the Atmospheric Imaging Assembly onboard the emph{Solar Dynamic Observatory}. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature (>8 MK) and density (sim1.0 imes109 cm-3), resulting in an enhanced emission measure (EM) over a broad temperature range (3 leq T(MK) leq 20). On the other hand, the CME LF has a relatively cool temperature (sim2 MK) and a narrow temperature distribution similar to the pre-eruption coronal temperature (1 leq T(MK) leq 3). The density in the LF, however, is increased by 2% to 32% compared with that of the pre-eruption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 leq T(MK) leq 4), but the density decreases by sim35% to sim40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection, (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region, and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption.

Authors: X. Cheng, J. Zhang, S. H. Saar, & M. D. Ding
Projects: SDO-AIA

Publication Status: 19 pages, 18 figures, 3 tables, accepted for publication in ApJ
Last Modified: 2012-10-30 09:51
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Investigation of the Formation and Separation of An EUV Wave from the Expansion of A Coronal Mass Ejection  

Xin Cheng   Submitted: 2011-12-19 18:17

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: X. Cheng, J. Zhang, O. Olmedo, A. Vourlidas, M. D. Ding, and Y. Liu
Projects: SDO-AIA

Publication Status: accepted for publication in The Astrophysical Journal Letters
Last Modified: 2011-12-20 07:51
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Observing Flux Rope Formation During the Impulsive Phase of a Solar Eruption  

Xin Cheng   Submitted: 2011-03-25 15:13

Magnetic flux rope is believed to be an important structural componentof coronal mass ejections (CMEs). While there exist much observationalevidence of the flux rope after the eruption, e.g., as seen inremote-sensing coronagraph images or in-situ solar wind data, thedirect observation of flux ropes during CME impulsive phase has beenrare. In this Letter, we present an unambiguous observation of a fluxrope still in the formation phase in the low corona. The CME ofinterest occurred above the east limb on 2010 November 03 withfootpoints partially blocked. The flux rope was seen as a bright blobof hot plasma in AIA 131Å passband (peak temperature ~11 MK) risingfrom the core of the source active region, rapidly moving outward andstretching upward the surrounding background magnetic field. Thestretched magnetic field seemed to curve-in behind the core, similarto the classical magnetic reconnection scenario in eruptive flares. Onthe other hand, the flux rope appeared as a dark cavity in AIA 211Åpasspand (2.0 MK) and 171Å passband (0.6 MK); in these relativelycool temperature bands, a bright rim clearly enclosed the dark cavity.The bright rim likely represents the pile-up of the surroundingcoronal plasma compressed by the expanding flux rope. The compositestructure seen in AIA multiple temperature bands is very similar tothat in the corresponding coronagraph images, which consists of abright leading edge and a dark cavity, commonly believed to be a fluxrope.

Authors: X. Cheng, J. Zhang, Y. Liu, M. D. Ding
Projects: SDO-AIA

Publication Status: 15 pages, 5 figures, accepted for publication in ApJ Letters
Last Modified: 2011-03-25 15:20
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A Comparative Study of Confined and Eruptive Flares in NOAA AR 10720  

Xin Cheng   Submitted: 2011-03-10 11:16

We investigate the distinct properties of two types of flares: eruptive flares associated with CMEs and confined flares without CMEs. Our sample of study includes nine M and X-class flares, all from the same active region (AR), six of which are confined and three others are eruptive. The confined flares tend to be more impulsive in the soft X-ray time profiles and show more slender shapes in the EIT 195 A images, while the eruptive ones are of long-duration events and show much more extended brightening regions. The location of the confined flares are closer to the center of the AR, while the eruptive flares are at the outskirts. This difference is quantified by the displacement parameter, the distance between the AR center and the flare location: the average displacement of the six confined flares is 16 Mm, while that of eruptive ones is as large as 39 Mm. Further, through nonlinear force-free field extrapolation, we find that the decay index of the transverse magnetic field in the low corona (~10 Mm) have a larger value for eruptive flares than that for confined one. In addition, the strength of the transverse magnetic field over the eruptive flare sites is weaker than that over the confined ones. These results demonstrate that the strength and the decay index of background magnetic field may determine whether or not a flare be eruptive or confined. The implication of these results on CME models is discussed in the context of torus instability of flux rope.

Authors: X. Cheng, J. Zhang, M. D. Ding, Y. Guo, and J. T. Su
Projects: None

Publication Status: 23 pages, 8 figures, 2 tables, ApJ in press
Last Modified: 2011-03-10 19:24
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Re-flaring of a Post-Flare Loop System Driven by Flux Rope Emergence and Twisting  

Xin Cheng   Submitted: 2010-05-11 19:07

In this letter, we study in detail the evolution of the post-flare loops on 2005 January 15 that occurred between two consecutive solar eruption events, both of which generated a fast halo CME and a major flare. The post-flare loop system, formed after the first CME/flare eruption, evolved rapidly, as manifested by the unusual accelerating rise motion of the loops. Through nonlinear force-free field (NLFFF) models, we obtain the magnetic structure over the active region. It clearly shows that the flux rope below the loops also kept rising accompanied with increasing twist and length. Finally, the post-flare magnetic configuration evolved to a state that resulted in the second CME/flare eruption. This is an event in which the post-flare loops can re-flare in a short period of sim16 hr following the first CME/flare eruption. The observed re-flaring at the same location is likely driven by the rapid evolution of the flux rope caused by the magnetic flux emergence and the rotation of the sunspot. This observation provides valuable information on CME/flare models and their prediction.

Authors: X. Cheng, M. D. Ding, Y. Guo, J. Zhang, J. Jing, T. Wiegelmann
Projects: None

Publication Status: accepted for publication in ApJ Letter
Last Modified: 2010-05-12 10:38
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Study the build-up, initiation and acceleration of 2008 April 26 coronal mass ejection observed by STEREO  

Xin Cheng   Submitted: 2010-02-26 04:28

In this paper, we analyze the full evolution, from a few days prior to the eruption to the initiation, and the final acceleration and propagation, of the CME that occurred on 2008 April 26 using the unprecedented high cadence and multi-wavelength observations by STEREO. There existed frequent filament activities and EUV jets prior to the CME eruption for a few days. These activities were probably caused by the magnetic reconnection in the lower atmosphere driven by photospheric convergence motions, which were evident in the sequence of magnetogram images from MDI (Michelson Doppler Imager) onboard SOHO. The slow low-layer magnetic reconnection may be responsible for the storage of magnetic free energy in the corona and the formation of a sigmoidal core field or a flux rope leading to the eventual eruption. The occurrence of EUV brightenings in the sigmoidal core field prior to the rise of the flux rope implies that the eruption was triggered by the inner tether-cutting reconnection, but not the external breakout reconnection. During the period of impulsive acceleration, the time profile of the CME acceleration in the inner corona is found to be consistent with the time profile of the reconnection electric field inferred from the footpoint separation and the RHESSI 15-25 keV HXR flux curve of the associated flare. The full evolution of this CME can be described in four distinct phases: the build-up phase, initiation phase, main acceleration phase, and propagation phase. The physical properties and the transition between these phases are discussed, in an attempt to provide a global picture of CME dynamic evolution.

Authors: X. Cheng, M. D. Ding, J. Zhang
Projects: STEREO

Publication Status: ApJ (in press)
Last Modified: 2010-02-26 18:52
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Abstracts by Author
Origin and Structures of Solar Eruptions I: Magnetic Flux Rope (Invited Review)
On the Characteristics of Footpoints of Solar Magnetic Flux Ropes during the Eruption
Spectroscopic Diagnostics of Solar Magnetic Flux Ropes Using Iron Forbidden Line
A Two-ribbon White-light Flare Associated with a Failed Solar Eruption Observed by ONSET, SDO, and IRIS
Extreme Ultraviolet Imaging of Three-dimensional Magnetic Reconnection in a Solar Eruption
Predicting the Arrival Time of Coronal Mass Ejections with the Graduated Cylindrical Shell and Drag Force Model
A Type II Radio Burst without a Coronal Mass Ejection
Imaging and Spectroscopic Diagnostics on the Formation of Two Magnetic Flux Ropes Revealed by SDO/AIA and IRIS
On the Relationship Between a Hot-channel-like Solar Magnetic Flux Rope and its embedded Prominence
Formation of a Double-decker Magnetic Flux Rope in the Sigmoidal Solar Active Region 11520
Differential Emission Measure Analysis of A Limb Solar Flare on 2012 July 19
Tracking the Evolution of A Coherent Magnetic Flux Rope Continuously from the Inner to the Outer Corona
Investigating Two Successive Flux Rope Eruptions In A Solar Active Region
The Driver of Coronal Mass Ejections in the Low Corona: A Flux Rope
Differential Emission Measure Analysis of Multiple Structural Components of Coronal Mass Ejections in the Inner Corona
Investigation of the Formation and Separation of An EUV Wave from the Expansion of A Coronal Mass Ejection
Observing Flux Rope Formation During the Impulsive Phase of a Solar Eruption
A Comparative Study of Confined and Eruptive Flares in NOAA AR 10720
Re-flaring of a Post-Flare Loop System Driven by Flux Rope Emergence and Twisting
Study the build-up, initiation and acceleration of 2008 April 26 coronal mass ejection observed by STEREO
A statistical study of the post-impulsive-phase acceleration of flare-associated coronal mass ejections

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